1. Field
The present invention relates to a light guide plate and a surface light source device, and specifically relates to a surface light source device for efficiently making light incident to a light guide plate having a smaller thickness than a height of a light source, and to a light guide plate, and the like.
2. Related Art
In recent years, with a reduction in a thickness of a mobile device that has a surface light source device, a reduction in a thickness of the surface light source device is also being required. To reduce the thickness of the surface light source device, it becomes necessary to reduce a thickness of a light guide plate. However, even when the thickness of a planar light guide plate can be reduced, there is a limit to a reduction in a height of a light source composed of an LED. Therefore, when a thin planar light guide plate is used, the height of the light source becomes larger than a thickness of an end surface (a light-incident end surface) of the light guide plate, and the light source arranged opposite to the light-incident end surface of the light guide plate is protruded to above an upper surface of the light guide plate. When the light source is protruded to above the light guide plate, all of light emitted from the light source does not enter the light incident end face of the light guide plate but a part of the light leaks to the outside of the light guide plate, and light utilization efficiency becomes poor.
FIG. 1A is a perspective view schematically illustrating a conventional surface light source device. A surface light source device 11 in FIG. 1A is configured by a light source 12 and a light guide plate 13, and the light source 12 is arranged opposite to a light-incident end surface 19 of the light guide plate 13. The light guide plate 13 is provided by integrally forming a light guide-plate main body 15 having substantially a uniform thickness and a light introducing portion 14 having a wedge shape. An inclined surface 16 is formed on an upper surface of the light introducing portion 14, and a directional transform pattern 17 (light-leakage prevention patterns) formed of a plurality of V-grooves 18 is formed on the inclined surface 16. A thickness of an end surface (the light-incident end surface 19) of the light introducing portion 14 is larger than a height of the light source 12. As a surface light source device that uses a light guide plate having such a structure, there is a one disclosed in Japanese Unexamined Patent Publication No. 2006-171253, for example.
In the surface light source device 11, light emitted from the light source 12 is incident from the light-incident end surface 19 into the light introducing portion 14. Since the thickness of the end surface (the light-incident end surface 19) of the light introducing portion 14 is larger than the height of the light source 12, the light emitted from the light source 12 is efficiently taken into the light introducing portion 14. The light incident to the light introducing portion 14 is reflected by the upper surface (the inclined surface 16) or a lower surface of the light introducing portion 14, is guided to the light guide-plate main body 15, and is emitted to an outside from a light emitting surface 20 of the light guide-plate main body 15.
In this case, when the inclined surface 16 is flat, there is a risk that a part of the light that is incident to the light introducing portion 14 leaks to an outside by being transmitted through the inclined surface 16, without being reflected by the inclined surface 16. Therefore, in the surface light source device 11, light leakage from the inclined surface 16 is reduced by providing the directional transform pattern 17 on the inclined surface 16. That is, in the surface light source device 11, because the V-grooves 18 as shown in FIG. 1B are provided on the inclined surface 16, an incidence angle of the light incident to the V-grooves 18 becomes large like light L indicated by solid line arrows, and the light L is reflected by the V-grooves 18 and does not easily leak from the inclined surface 16. As a result, according to the surface light source device 11 in such a structure, it becomes possible to improve light utilization efficiency and reduce the thickness of the surface light source device.
However, even when the directional transform pattern 17 formed of the plurality of V-grooves 18 is provided on the inclined surface 16, light leakage from the inclined surface 16 occurs as shown in FIG. 1B (of the light L that is incident to the light introducing portion 14, the light that leaks to an outside is indicated by broken-line arrows). FIG. 1B is a schematic view illustrating behaviors of the light L in high light intensity that is emitted from a light emission center 12a. Of the light emitted from the light emission center 12a, the light L emitted substantially forward from the light emission center 12a is reflected from both a groove slant surface 18a that faces a center surface C (a flat surface that passes through the light emission center C of the light source and is perpendicular to a light-incident end surface 19 and a light emitting surface 20 is referred to as the center surface C) and a groove slant surface 18b that faces opposite to the center surface C as indicated by solid-line arrows, and is not easy to leak from the inclined surface 16. On the other hand, in the surface light source device 11, the directional transform pattern 17 has V-grooves of the same shapes having bilaterally symmetrical cross-sectional shapes that are repeatedly arranged, as shown in FIG. 1B. Therefore, the light L obliquely emitted from the light emission center 12a becomes easy to leak to an outside as indicated by the broken-line arrows. That is, when a position where the light is incident to the directional transform pattern 17 becomes far from the center surface C, the light L becomes incident to the groove slant surface 18b, on which a normal line faces opposite to the center surface C, at an angle gradually nearer perpendicular to the groove slant surface 18b. Accordingly, the light L easily leaks from the groove slant surface 18b. 